This invention relates to surface modification of the slip characteristics of the inside diameter (ID) surface of tubing composed of polymeric materials such a silicone rubber, polypropylene, polyethylene, polyvinylchloride, fluoropolymers and the like or other dielectric materials and to improved methods and apparatus for effecting such modifications.
Polymeric plastic tubing, particularly that of small diameter, and most especially that of silicone rubber, is used in many medical applications and devices. Actually, silicone rubber (especially peroxide cross linked silicone elastomer with silica filling) is the polymer of choice for tubing in many medical applications involving implantation. In may instances this tubing is less than about 2 mm in ID.
Although this invention is applicable to other polymeric materials and dielectric materials, it will be described herein with particular reference to silicone rubber, the preferred embodiment. For example, the so called "screw-in" pacing leads make use of very small diameter tubing such as less than 0.055 inch (OD) with an ID of 0.035 inch. In this type of lead, an elongate wire core (usually in the form of a coil) having a helical screw-in electrode at its distal end is placed inside an elongate silicone, tube to provide a catheter-like device. The core wire is manipulated at the proximal end of this arrangement by the physician during implantation to screw the helical electrode into heart tissue and fix the lead in place. Of course, the lead involves other structure not described here for simplicity. Also, this is merely one example of many lead structures which include silicone tubing or the like.
Unfortunately, silicone rubber has a tacky surface, which causes excessive friction, making placement of a core wire in such small diameter tubing difficult from the production standpoint in the first place. The ease of placing a core wire or the like in such tubing is referred to as "stringability". In the second place, these friction characteristics also make torque transfer through the tubing difficult thus, for example, making difficult the turning of the core wire which is preferably a torsion coil in the aforementioned "screw in" pacing lead to screw the helical electrode into tissue. Thirdly, due to sticking of the core wire to the inside of the tubing, flex life is shortened.
Previous practices to ameliorate these friction characteristics have involved; 1) the use of harder materials which are more slippery but less biostable and less suitable for implantation e.g., Polyurethane, 2) coating, 3) hardening, 4) swelling and, even 5) the use of environmentally unfriendly materials such as chlorofluorocarbons (CFC). Also, plasma discharge has been used on tubing with some degree of success in this effort. However, none of these practices have been satisfactory with respect to long lengths of tubing and the provision of a uniform surface therein or with respect to successfully modifying the relatively small ID polymer tubing with which this invention is most particularly concerned i.e., less than 1 mm ID.
More specifically, it is known in the art of plasma discharge that exposure of polymeric surfaces to such discharge is effective in modifying the surface to improve its slip characteristics. It is also known to apply this phenomena to plastic polymeric tubing. U.S. Pat. No. 5,133,422 is directed to improving the slip characteristics of such tubing on its OD. U.S. Pat. No. 4,692,347 is directed to plasma deposition of coatings and to improving blood compatibility on both the OD and the ID surfaces of polymeric tubing by coating it under discharge conditions in a single chamber. Primarily, this technique is successful with tubing of about 3 mm to 6 mm in diameter (more specifically tubing with a length to diameter ratio of 100 or less) but it is not effective with relatively small tubing such as tubing less than 1 mm ID, either for discharge in an inert gas e.g., N.sub.2, which is a non-film forming gas and no coating, and provides just surface modification of slip characteristics, or for discharge in an inert gas of the film forming or coating type. Film forming gasses are generally a volatile monomer such as methane or any hydrocarbon gas or siloxane vapor or the like which modify surface chemistry even more by forming a coating. However, to date, the literature has not provided examples of plasma discharge treatment of the ID of extremely small tubing, e.g., less than 1 mm ID with either type of gas.
The theory and practice of radio frequency (RF) gas discharge is explained in detail in 1) "Gas-Discharge Techniques For Biomaterial Modifications" by Gombatz and Hoffman, CRC Critical Reviews in Biocompatibility, Vol. 4, Issue 1 (1987) pp 1-42; 2) "Surface Modification and Evaluation of Some Commonly Used Catheter Materials I Surface Properties" by Trials and Andrade, Journal of Biomedical Materials Research., Vol. 17, 129-147 (1983), and 3) "Surface Modification and Evaluation of Some Commonly Used Catheter Materials, II. Friction Characterized" also by Trials and Andrade, Journal of Biomedical Materials Research, Vol. 17, 149-165 (1983). All of the foregoing is incorporated herein by reference. For purposes of this invention, the gas discharge process or radio frequency discharge as contemplated herein need only be such as to give rise to a plasma glow discharge which interacts with surfaces exposed thereto, such as silicone rubber, to alter same by reaction therewith.
A number of patents have been reviewed in which plasma reactors are disclosed and which can, to a degree, generate some plasma within small diameter tubing. However, the smallest inside diameter that these reactors have been able to generate a slow discharge within is about 3-4 mm (0.118-0.158 inches). Most of these methods depend on gas flow through the ends of the tubing and controlled pressures within the tubing. Some create the plasma outside and cause it to flow into the ends of the tubing (U.S. Pat. Nos. 5,244,654 and 4,752,426). All of them use continuous wave energy (RF or microwave) to excite the plasma.
______________________________________ LIST OF U.S. PATENTS ______________________________________ U.S. Pat. No. 5,244,654 09/14/1993 Narayanan U.S. Pat. No. 5,223,308 06/29/1993 Doehler U.S. Pat. No. 5,133,986 07/28/1992 Blum et al. U.S. Pat. No. 4,948,628 08/14/1990 Montgomery et al. U.S. Pat. No. 4,927,676 05/22/1990 Williams et al. U.S. Pat. No. 4,846,101 07/11/1989 Montgomery et al. U.S. Pat. No. 4,752,426 06/21/1988 Cho U.S. Pat. No. 4,718,907 01/12/1988 Karwoski et al. U.S. Pat. No. 4,692,347 09/08/1987 Yasuda U.S. Pat. No. 4,448,954 12/18/1984 Hatada et al. U.S. Pat. No. 4,261,806 04/14/1981 Asai et al. ______________________________________
Some of the patents (U.S. Pat. Nos. 4,752,426 and 4,261,806) cite heat build-up and thermal degradation of the tubing as a problem to be overcome. To prevent tubing over heating, one of the patents (U.S. Pat. No. 4,261,806) transports the tubing through an oil bath, which has obvious disadvantages.
It is a primary object of this invention to provide polymeric tubing which exhibits much improved slip characteristics. This and other objects will be clear from the following description.